Simple reqwuest, but one that would save me time:
When you attach a canvas/invas to model against, you then go in to "calibrate" the canvas based on real world measurements. My suggestion would be to add on option to the calibrate tool that says "Align the canvas to an axis to the midpoint of the measurement".
Right now, one has to eyeball the canvas as having its midpoint exactly on any given axis...when you are turning on symmetry and modeling it can become apparent that the attached canvas is slightly off, and you have to go in an readjust. When you measure the canvas, take the midpoint of that measurement and align it to a selected axis, and all those issues are solved.
Thanks!